A former eco-terrorist tied to a number of arsons in Michigan, including the 2003 burnings of two upscale houses under construction in Washtenaw County, has gotten more than three years trimmed off his prison sentence thanks to his continued cooperation with authorities.

Frank Ambrose, 27, a one-time radical environmentalist turned government informant, was rewarded again for his substantial assistance to the government, which means he's apparently still helping authorities while in prison, the Associated Press reported. U.S. District Judge Paul Maloney didn't elaborate Monday while cutting Ambrose’s sentence from nine years to just under six years, the AP said.

Ambrose and his former wife, Marie Mason, were involved in the March 21, 2003, arson of two homes under construction in the Mystic Ridge subdivision off of Plymouth Road in Superior Township, investigators said. Homes in the subdivision were priced at $500,000 to $700,000 at the time.

Authorites say Mason and Ambrose -- who have since divorced -- were affiliated with the Earth Liberation Front and set the fires to protest urban sprawl. "ELF" and "no sprawl" were sprayed in black on a nearby home under construction.

The couple also were eventually convicted of setting a fire that caused $1 million in damage to a building at Michigan State University in 1999 to protest research on genetically modified plants.

The FBI didn't catch Ambrose until 2007 when agents got an extraordinary break: A man foraging for wood in a Detroit-area trash bin found Ambrose's writings, a gas mask, an M-80 explosive and other possessions.

Ambrose subsequently became an informant, cooperation that helped when he was first sentenced in 2008. The prosecutor at the time called his assistance "nothing short of remarkable." Ambrose had turned on his wife and secretly recorded conversations at gatherings of radical environmental groups.

No formal criminal charges were ever filed in the Washtenaw County arsons, but Ambrose admitted to burning the houses as part of his guilty plea in 2008 to the MSU arson. The government estimated the damage to the houses at $1 million in court filings.

Ambrose has been in federal prison since 2008.

In 2009, Mason was sentenced to nearly 22 years in prison for the MSU arson.

The prosecutor and Ambrose's attorney have made five sealed filings in the case since January. The Associated Press recently asked the Kalamazoo-based judge to make the documents public, but he declined.

"The interests of confidentiality outweigh the interests of this stuff being public," Assistant U.S. Attorney Hagen Frank said Wednesday, declining further comment.

You may reach Steve Pepple at stevepepple@annarbor.com. The Associated Press contributed to this report.